Thermocouple tube and protective coating



c. A. RAGSDALE 3,010,480

THERMOCOUPLE TUBE AND PROTECTIVE coATING Filed Oct. 13, 1958 F/gz Nov.28, 1961 1N V EN TOR.

' CLIFFORD A. RAGSDALE MA1-) Kari' maf 3,010,480 THERMOCOUPLE TUBE ANDPRTECTIVE CQA'IING Clifford A. Ragsdale, 214 Connecticut Ave., Lorain,Ghia Filed Oct. 13, 1958, Ser. No. 767,000 4 Claims. (Cl. 13S-89) Theinvention relates in general -to thermocouple tubes and a protectivecoating therefore which coating may also be utilized as a protectivecoating for other objects and devices which are subject to the same orsimilar conditions in their ultimate use.

Referring specifically to thermocouple and/or pyrometer tubes, it mightbe pointed out that they Vare used to contain and protect the means forindicating temperature (for example dissimilar wires in thermocouples)when the devices are subjected to a corrosive environment in theirultimate use. Thermocouple tubes in use today for immersion in aluminum,lead and zinc for example, are primarily constructed of cast iron orsteel pipe closed at one end and open at the other. When these tubes aresuspended substantially vertically in a container of molten metal themovement of the heated metal along with the corrosive nature of thematerial itself causes the closed end of the tube to fail first, washingout in a concave fashion. The next failure occurs in the cylindricalwall portion of the tube. The physical construction of the thermocoupletube of the present invention along with the protective coatings andmodifications thereof enables the tube herein disclosed to stand upunder operating conditions up to fifty times as long as standard ironand steel tubes. With the protective coatings herein disclosed, used onconventional iron and steel tubes, the

life of the same have been increased as much as fifteen to twenty times.

It is therefore an object of this invention to provide a new andimproved thermocouple tube.

Another object of the invention is to provide a thermocouple tube havingan end thereof which comprises a solid, substantially conical portionwhich is a separate unit Ifrom the thermocouple wires when installed.The solid conical portion serves to provide a surface which is at asmall angle with respect to the normal ilow of metal. This serves toprevent washout which occurs in the conventional thermocouple tube.

Another object of the invention is to provide a new and novel method ofmaking a thermocouple tube.

Another object of the invention is to provide a protective coating andmethod of making the same which includes a first coating ofnickel-chromium with a fluxing agent, a second coating ofnickel-chromium, and a third coating of aluminum or zirconium oxide withan aluminum or zirconium oxide or an aluminum sealer. When the articleor device is to 4be immersed in a molten metal it is also preferablyprovided with a fourth coating of the molten metal.

Another object of the invention is to provide a protective coating andmethod of making the same which includes a first coating ofnickel-chromium and a second coating of aluminum oxide with a sealer ofaluminum oxide or aluminum or of zirconium oxide with a sealer ofzirconium oxide or aluminum.

FIGURE l is a view of a steel or iron pipe partially in section and aclosure member which is utilized in constructing the thermocouple tubeof the present invention;

3,0l0,480 Patented Nov. 28, 1961 'ice FIGURE 2 shows the closure memberwelded in place in an open end of the pipe;

FIGURE 3 is a view similar to FIGURE 2` but showing the closure membermachined to a generally conical shape and the weld being machinedsmooth;

FIGURE 4 is a view of the thermocouple tube of the present inventionwith the protective coatings thereon and indicating the tube in avertical position which is its normal position within a container ofmolten metal;

FIGURE 5 is an enlarged view taken generally along the line 5 5 ofFIGURE 4 and showing all four of the protective coatings slightlyexaggerated -as compared to the pipe thickness for better ease inillustra-ting the same;

FIGURE 6 is a fragmentary View similar to that shown in FIGURE 5 butshowing a modification thereof in Ithe sense that the fourth oroutermost coating has been dispensed with; and

FIGURE 7 is a View similar to FIGURE 6 ybut illustrating a furthermodification in that the rst or innermost protective coating adjacentthe surface of the pipe, and the fourth or outermost coating have beendispensed with.

The preferred method of making the thermocouple tube 10 of the presentinvention is best illustrated and seen in FIGURES 1-5. FIGURE lillustrates the step of selecting a piece of pipe 12, which may also bercferred lto as an annular member, and which has iirst and second openends 13 and 14, respectively. The pipe is also provided with inside andoutside surfaces 16 and 17 respectively, which have inside and outsidediameters. The pipe 12 is preferably of a steel or cast ironconstruction. A closure member 19 is also selected and is preferably ofa steel or cast iron construction and this closure member includes alirst cylindrical portion 20 ywhich has a diameter which issubstantially equal to the outside diameter of the pipe. The closuremember `also has a second cylindrical portion 21 which is smaller thanthe first and ywhich has a diameter substantially equal to the insidediameter ofthe pipe. The closure member is preferably initially acylindrical member having a single diameter substantially equal to theirst cylindrical portion and the second cylindrical portion is providedpreferably by machining or removing a part o-f the material. As a resultof this construction, the iirst and second cylindrical portions can besaid to be integrally connected together and this type of constructionforms a shoulder 23 therebetween. The next step involved in making thethermocouple tube of the present invention is seen in FIGURE 2 and thiscomprises placing the second cylindrical portion 21 of the closuremember into the first open end 13 of the pipe with the shoulder 23abutting .the iirst end of the pipe. The closu-re member is then weldedto the first end of the pipe as at 24. The first cylindrical portion o-fthe closure member 19 is next machined to a substantially conical shapewhich is sho-Wn in FIGURE 3. The weld is also machined smooth with theoutside surface of the pipe. This machining is accomplished by anymethod well known in the art and a tool 25 has been indicatedschematically in FIGURE 2. The second end portion of the pipe isprovided with threads 27 which may be provided thereon at nearly anystage in the making of the device. The threads are, however, preferablyput on the second end portion of the pipe before the closure member isatiixed to the first end portion.

The -next steps in making the thermocouplle tube of the rosion anderrosion resistant barrier.

present invention involves the applying of the successive protectivecoatings of the invention to the surface of the thermocouple tube. Thethermocouple tube at this stage includes the assembled elements; namely,the pipe 12 and the closure member 19. The tube is cleaned by any methodwhich'is necessary to remove dirt or foreign material from the outsideof the same. This` can either be by physical methods such as wirebrushing, wiping or by suitable chemical means if necessary. After thetube has been thoroughly cleaned, -it is grit blasted by means which arewell recognized in the art. This serves to in effect roughen up theoutside of the thermocouple tube. A nickelechromium alloy is thensprayed on the outside surface of the grit blasted tube. Thisnickel-chromium alloy contains suitable materials which act as a flux.The essential yelements contained in this alloy for acting as a flux areboron and silicon. The exact composition of an alloy which is suitablefor this first coating comprises the following composition; namely, 15%chromium, 75% nickel, 3% silicon, 3% boron, 1% carbon, and 3% iron. Thisfirst coating is indicated by the reference numeral 30 (FIGURE 5) and ispreferably on the order of .015 inch. This first protective coating 30is applied by what is known in the art as metal spraying (metallizing),and

Vthe mechanics of this means of application are well known by thoseskilled in the art. A brief discussion, however of the means of applyingand the apparatus used in this art are in order at this time. What iscommonly referred to as a metallizing gun, is utilized in applying thisfirst lcoating and this is primarily a device which has a hopper orcanister located over the gun which contains the materials to beapplied. These materials are in a finely divided or powder form and aregenerally gravity` fed through a metering tube into the tiring nozzle.This nozzle rin construction, is quite Vsimilar to a gas fired cuttingtorch nozzle such as is used in an acetylene cutting torch. The powderis fed through the center of the nozzle with the llame being fed throughsmall hotles located near the outer rim of fthe nozzle which isgenerally circular in shape. As the material, which is in powder formenters the flame, it is melted 'and propelled to the object to becoated, which is in this case the thermocouple tube.

Another type of metallizing gun which may be used in producing theresults ofthe present invention employes an air or an electric motor todrive two notched wheels, one located above the other. The material tobe applied, which is in Wire form passes between these two notchedwheels which serves to move the wire material intothe tiring nozzle. l

After this first coating 30 has been applied, it is heat treated atapproximately 1900 F. and because of the use of the uxing material inthe first coating, the rst coating flows or becomes quite smooth andconforms very closely to the base material of the thermocouple tube. Asan `attempted explanation, but without limitation, it is believed thatthis irst coating creates in substance a oor- It also provides a casing,in effect, which serves to correlate the expansion between thethermocouple tube and other coatings which are to be applied and whichwill be described hereinafter.

After the iirst coating has been completed, the thermocouple tube ispreferably cooled and it again is grit blasted. After this yhas beenaccomplished a second coating 3-1 is applied by the same means as thefirst coating is applied namely by utilizing a metallizing gun. Themetals of this coating comprise an alloy of nickel and chromium, thepreferred mixture being substantially 75% nickel and 25% chromium. Thiscoating is applied to a thickness on the order of .010 inch. Thiscoating does not contain any iluxing agent and as a result, produces aprotective coating which acts to provide a cushion between the firstcoating and the thermocouple tube and a third coating 32 which isapplied land will be described hereinafter. This coating appears tocompensate for the difference in coeicient of expansion between thethermocouple tube and the third coating. Y

The third coating 32 is next applied to the thermocouple tube on top ofthe 'second coating 31. If the thermocouple tube is to be used inaluminum, lead or zinc, for example, or any other mass of molten met-alof a temperature up to about 2600 F., the third coating comprises 1analuminum oxide. This material is app-lied as discussed hereinabove beingoriginally in a powder form and being sprayed through the metallizinggun. This coating of laluminumoxide is preferably applied to a thicknesson the order of .020 inch. Where the thermocouple tube is to besubjected to temperatures of about 2600 F. and above, a differentmaterial is preferably utilized which will'be discussed hereinafter. Itwould be well to point out at thisv time that the third coating 32 of ofaluminum oxide, when applied in this hereinabove described manner, to acertain extent is porous. Y Molten metals, especially aluminum, lead andzinc, will penetrate the aluminum oxide coating and attack the metalsunderneath, which eventually leads to the destruction of these metalsand a thermocouple tube failure. Therefore, it is of importance that theporosity be eliminated as much las possible. To accomplish this end, asealer is utilized which comprises aluminum oxide suspended in asuitable vehicle such as lacquer or any material that will burn olf orVevaporate without creating an undesirable residue. This sealer materialpenetrates the somewhat porous surface of the third coating and whensubjected to any heat, forms a film that greatly decreases Y theporosity. c

In the event the thermocouple tube or other obJects are to be subjectedto temperatures of about 2600 F.

Vand above the third coating, instead of Acomprising of aluminum oxide,comprises Ya zirconium oxide. The

is substantially the same and the material is applied in substantiallythe same manner when the third coating is of zirconium oxide. The sealerwhich is Vpreferably used, comprises zirconium oxide carried by asuitable vehicle such as lacquer. Water is also a suitable vehicle forcarrying the aluminumand zirconium oxide Sealers.

After the third coating 32 has been applied, which includes the sealer,a fourth coating 33 is applied. Before this coating is applied, thesolvent or carrier for the sealer mentioned above is preferably removed.This isreadily accomplished by applying heat to the third coating suchas by a torch or by placing the thermocouple tube in a heat treatingoven toremove the carrier. If the fourth coating is not to be applied,as will be described hereinafter, then the removing of the solvent orcarrier can be dispensed with unless the same would contaminate a moltenmetal mass intowhich it is to be immersed. The

sea-ler is best applied by either brushing on, dipping or spraying. AThe fourth protective coating which is applied, is applied primarilywhen the thermocouple tube or other I device to be protected is to beimmersed in a molten metal bath. This fourth coating-comprisespreferably the metal into which it is to be immersed in its nal use. Inother words, if the thermocouple tube is going to be yused in moltenaluminum, then aluminum is preferably sprayed on after the fourthcoating.- This material is applied preferably in the same manner as theabove mentioned rst, second and third protective coatings. If the `tubeis to be used in lead, then a lead coating would It has beenfound,'however, that Satisfactory results` can be obtained Without theuse of the fourthV coating 33 in some instances particularly where thethermocouple tube is not to be subjected to the action of molten metalsbut only to a very corrosive atmosphere. In this instance then, thethermocouple tube and protective coating would be such as shown inFIGURE 6 and only coat-ings 3i), 31 and 32 would be utilized.

A modification of the protective coating structures has produced verygood results and FIGURE 6 may be resorted to in describing thismodification. In this modification, the first and second coatings 3) and31 are as described hereinabove and also as described hereinabove, thethird coating may comprise either aluminum or zirconium oxide. Theessential difference in this modification is in the sealer. In thisinstance, the sealer which is utilized is aluminum rather than aluminumoxide and `is preferably in the powder form. This aluminum is suspendedin a suitable vehicle or carrier as described hereinabove, which carrierwill burn off or be disposed of when heated. A suitable example of sucha carrier which might be utilized is aluminum paint, and aluminum powdermay be added to the paint, to provide the proper concentration `of thealuminum in the vehicle or carrier. It has been found that athermocouple tube or other article which has been coated in -this mannercan be heated to 2600 F. and over and immersed at this heat into cold`water with substantially no ill effects upon the coating. The aluminumor zirconium oxide of the third coating will withstand this thermalshock with no damage to this coating and no fractures whatever to thecoating. This coating may be 'reheated a number of times with no effectwhatever. if the sprayed third coating of aluminum or zirconium oxide isutilized without this sealer and is heated to the temperature mentionedand then immersed in cold water, the coating will fracture and fly off.If it is exposed to this temperature in a corrosive atmosphere and keptat this temperature for a period of time, it will deterioratecompletely. lf the sealer, such as described hereinabove, is utilized,the coating will not deteriorate and will withstand this temperature.'It is sometimes desirable to heat the third coating after the sealerhas been applied to drive oft the vehicle or carrier. However, this maybe dispensed with whenever the vehicle will evaporate of its owninitiative or if it will not be detrimental to the material into whichit is to be immersed.

It has also been found that a thermocouple tube, covered with only twoprotective coatings as shown in FIGURE 7, will provide the device `witha life which is many times the life of the ordinary present daythermocouple tubes. The two coatings which may be satisfactorilyutilized with beneficial results are the coatings 31 and 32 which havebeen described hereinabove and which have been illustrated in FIGURES 5and 6. In other words, the nickel-chromium alloy, without a fiuxingagent or agents is applied as the first coating on the outside of thetube. The second coating 32 would then comprise either aluminum orzirconium oxide. suitable sealer would be utilized and these sealershave been appropriately described hereinabove. To repeat, however, thealuminum oxide coating with the aluminum oxide sealer may be utilizedand the zirconium oxide coating with the zirconium oxide sealer may beused. The alternate sealer of aluminum may be utilized for both thealuminum oxide and zirconium oxide coatings. This coating does notproduce as good results as do the coatings described in conjunction withFIGURES 5 and 6. This type of protective coating does, however, producevery marked results from anything which is at all known or available atthe present time.

The protective coatings are applied to the thermocouple tube from theconical tip of the same back to the point at which the threads A27begin. The thermocouple wires which are actually utilized to determinethe tempera-ture of a molten bath or for other systems, do not form apart or are not secured to any of the elements shown in the drawings.The wires are, however, inserted into the tube and may bear against theend of the closure Thena r member. The thermocouple tube is preferablymounted in a vertical manner in the upper portion of a bath of moltenmetal. Referring to FIGURE 4, this would be the general direction whichthe thermocouple tube would be immersed into a pot or bath of moltenmetal and would assume this vertical position. The heating means in sucha bath would preferably be located at the lowermost vertical position ofthe bath and as a result the natural flow of molten metal would be in avertical direction where it would become colder than the rest and wouldrecirculate again to the bottom of the bath. Due to this movement of themolten metal, the metal has a natural tendency to attack portions of thethermocouple tube which offer the greatest resistance to its movement.This action is greatest on the end or the tip of the thermocouple tubeand when a flattened or blunt end is presented to this material flowwhich end is substantially normal thereto, this offers a great deal ofresistance. Because of the construction of the herein disclosedthermocouple tube with the solid conical shaped end portion or tip, thisconical end will tend to deflect the molten metal as it rises upwardlythus eliminating as much as possible, any horizontal areas which offer aresistance to the movement of the molten metal. Even after an extendeduse of the instant thermocouple tube and after a breakdown of thecoating occurs, the life of the device is further enhanced due to theend of the tube being of the solid construction. This is because theerrosion in the normal thermocouple tube tends to wear the metal out ina concave fashion. The increased life is due to the increased length oftime which is required for the molten metal to wash out or errode theentire solid conical end of the tube. This solid conical portion, as faras dimensions are concerned, is usually never shorter than at least 3 or4 inches as compared to 1A to 3/s of an inch thickness for the ends ofstandard thermocouple tubes.

Although the protective coating of the present invention has beendescribed primarily in connection with thermocouple tubes, it will bereadily appreciated that such a coating has utility as a protectivecoating for any metals and materials which are to be used in corrosiveenvironments as described hereinabove. The protective coating, however,is very peculiarly adapted for use in connection with thermocoupletubes.

What is claimed is:

l. A thermocouple tube comprising a hollow cylindrical member havingfirst and second end portions, said first end portion being closed by agenerally conical shaped closure member, a corrosion resistant coatingon the outside of said tube including a first layer on said tube of analloy which comprises Cr and Ni and a fluxing agent, a second layer onsaid first layer of an alloy which comprises Ni and Cr, a third layer onsaid second layer, said third layer comprising a porous materialselected from the group consisting of aluminum and zircomum oxide and asealer of aluminum.

2. A thermocouple tube comprising a hollow cylindrical member having afirst and second end portions, said first end portion being closed by aconical shaped closure member, a corrosion resistant coating on theoutside of said tube including a first layer on said tube of an alloywhich comprises Cr and Ni, a second layer on said rst layer, said secondlayer comprising a porous zirconium oxide with the porous zirconiumoxide being filled with zirconium oxide.

3. A corrosion resistant article comprising a member having a firstprotective coating of chromium and nickel with a uxing agent, a secondprotective coating comprising chromium and nickel, a third protectivecoating comprising porous material selected from the group consisting ofzirconium and aluminum oxide with the pores sealed with aluminum.

4. A corrosion resistant article comprising a member having a firstprotective coating of chromium and nickel with a uxng agent, a secondprotective coating comprising chromium and nickel, a third protectivecoating comprising aluminum oxide.

References Cited in the le of this patent 5 UNITED STATES PATENTSChristman Nov. 15, 1904 McIlory May 12, 1914 McIlory June l5, 1937 10Simpson et al June 24,

1. A THERMOCOUPLE TUBE COMPRISING A HOLLOW CYLINDRICAL MEMBER HAVINGFIRST AND SECOND END PORTIONS, SAID FIRST END PORTION BEING CLOSED AGENERALLY CONCIAL SHAPED CLOSURE MEMBER, A CORROSION RESISTANT COATINGON THE OUTSIDE OF SAID TUBE INCLUDING A FIRST LAYER ON SAID TUBE OF ANALLOY WHICH COMPRISES CR AND NI AND A FLUXING AGENT, A SECOND LAYER ONSAID FIRST LAYER ON SAID SECOND WHICH COMPRISES NI AND CR, A THIRD LAYERON SAID SECOND LAYER, SAID THIRD LAYER COMPRISING A POROUS MATERIALSELECTED FROM THE GROUP CONSISTING OF ALUMINUM AND ZIRCONIUM OXIDE AND ASEALER OF ALUMINUM.